Variable font support by ramezgerges · Pull Request #3703 · mono/SkiaSharp

ramezgerges · 2026-04-20T21:17:51Z

Variable Font Support

Adds OpenType variable font APIs to SkiaSharp and HarfBuzzSharp, enabling runtime font variation axis control, typeface cloning with custom design positions, and full axis/instance querying.

Related skia PR: mono/skia#185

New Types

Type	Description
`SKFourByteTag`	Four-byte tag struct wrapping Skia's `SkFourByteTag`. Parse, ToString, implicit uint conversion.
`SKFontVariationAxis`	Generated struct: `Tag`, `Min`, `Default`, `Max`, `IsHidden`
`SKFontVariationPositionCoordinate`	Generated struct: `Axis` (tag), `Value` (float)
`SKFontArguments`	ref struct: `VariationDesignPosition`, `CollectionIndex` — accepts spans, arrays, stackalloc

New `SKTypeface` APIs

API	Description
`VariationDesignParameters`	Property → `SKFontVariationAxis[]` — all axes the font supports
`VariationDesignParameterCount`	Property → `int` — axis count for span pre-allocation
`GetVariationDesignParameters(Span<>)`	Span overload — allocation-free
`VariationDesignPosition`	Property → `SKFontVariationPositionCoordinate[]` — current axis values
`VariationDesignPositionCount`	Property → `int`
`GetVariationDesignPosition(Span<>)`	Span overload — allocation-free
`Clone(ReadOnlySpan<SKFontVariationPositionCoordinate>)`	Clone typeface with new variation position
`Clone(SKFontArguments)`	Clone with full control (position, collection index)

New HarfBuzzSharp `Face` APIs

API	Description
`HasVariationData`	Whether font has `fvar` table
`VariationAxisCount`	Axis count property
`VariationAxisInfos`	Property → `OpenTypeVarAxisInfo[]`
`GetVariationAxisInfos(Span<>)`	Span overload
`TryFindVariationAxis(Tag, out info)`	Find axis by tag
`NamedInstanceCount`	Named instance count property
`GetNamedInstanceDesignCoordsCount(int)`	Coord count for a named instance
`GetNamedInstanceDesignCoords(int)`	Design coords array
`GetNamedInstanceDesignCoords(int, Span<float>)`	Span overload
`GetNamedInstanceSubfamilyNameId(int)`	Name table ID for subfamily
`GetNamedInstancePostScriptNameId(int)`	Name table ID for PostScript name

New HarfBuzzSharp `Font` APIs

API	Description
`SetVariations(ReadOnlySpan<Variation>)`	Set variation values by tag
`SetVariationCoordsDesign(ReadOnlySpan<float>)`	Set design-space coordinates
`SetVariationCoordsNormalized(ReadOnlySpan<int>)`	Set normalized coordinates
`VariationCoordsNormalized`	Property → `int[]` — current normalized coords
`GetVariationCoordsNormalized(Span<int>)`	Span overload
`SetVariationNamedInstance(int)`	Select a named instance

Usage — Array-based

using var data = SKData.Create(File.OpenRead("Inter.ttf"));
using var typeface = SKTypeface.FromData(data);

// Query axes
foreach (var axis in typeface.VariationDesignParameters)
    Console.WriteLine($"{axis.Tag}: {axis.Min} .. {axis.Default} .. {axis.Max}");
// Output: wght: 100 .. 400 .. 900

// Clone with bold weight
var position = new[] {
    new SKFontVariationPositionCoordinate {
        Axis = SKFourByteTag.Parse("wght"),
        Value = 900
    }
};
using var bold = typeface.Clone(position);

Usage — Allocation-free with Span

// Pre-allocate based on count
Span<SKFontVariationAxis> axes = stackalloc SKFontVariationAxis[typeface.VariationDesignParameterCount];
typeface.GetVariationDesignParameters(axes);

// Clone with stackalloc
Span<SKFontVariationPositionCoordinate> pos = stackalloc SKFontVariationPositionCoordinate[] {
    new() { Axis = SKFourByteTag.Parse("wght"), Value = 900 }
};
using var bold = typeface.Clone(pos);

Console output

Axes for Inter:
  opsz: 14 .. 14 .. 32
  wght: 100 .. 400 .. 900

Tag round-trip: wght → 0x77676874 → wght

Visual comparison

Base (wght: 400)	Bold (wght: 900)

C API changes (companion PR in mono/skia)

sk_fourbytetag_t typedef matching SkFourByteTag
sk_fontarguments_variation_axis_t and sk_fontarguments_variation_position_coordinate_t structs
sk_typeface_get_variation_design_parameters(), sk_typeface_get_variation_design_position(), sk_typeface_clone_with_arguments()
DEF_MAP for layout-compatible struct conversion, static_assert in sk_structs.cpp

Tests

151 tests covering:

Exact axis values (tag, min, default, max, isHidden) for Distortable.ttf
SKFourByteTag: Parse, ToString, char constructor, implicit conversion, equality, edge cases (null, empty, short, long strings), known tag hex values, native round-trip
Span vs property equivalence for all overloads
Static font graceful handling (empty arrays, zero counts)
Clone preserves exact design position through native round-trip
Rendering tests: different weights produce different pixels, ink coverage varies, consistent clones match
HarfBuzz: axis queries, named instances, normalized coordinates, variation setting
Negative index validation on all instance methods

Gallery sample

Variable Fonts sample using Inter — weight and optical size sliders with weight spectrum comparison strip.

github-actions · 2026-04-20T21:18:00Z

📦 Try the packages from this PR

Warning

Do not run these scripts without first reviewing the code in this PR.

Step 1 — Download the packages

bash / macOS / Linux:

curl -fsSL https://raw.githubusercontent.com/mono/SkiaSharp/main/scripts/get-skiasharp-pr.sh | bash -s -- 3703

PowerShell / Windows:

iex "& { $(irm https://raw.githubusercontent.com/mono/SkiaSharp/main/scripts/get-skiasharp-pr.ps1) } 3703"

Step 2 — Add the local NuGet source

dotnet nuget add source ~/.skiasharp/hives/pr-3703/packages --name skiasharp-pr-3703

More options

Option	Description
`--successful-only` / `-SuccessfulOnly`	Only use successful builds
`--force` / `-Force`	Overwrite previously downloaded packages
`--list` / `-List`	List available artifacts without downloading
`--build-id ID` / `-BuildId ID`	Download from a specific build

Or download manually from Azure Pipelines — look for the nuget artifact on the build for this PR.

Remove the source when you're done:

dotnet nuget remove source skiasharp-pr-3703

Copilot

Pull request overview

This PR adds OpenType variable font (font variations) support to the managed SkiaSharp and HarfBuzzSharp APIs, including new querying/cloning APIs on SKTypeface, variation-axis APIs on HarfBuzzSharp.Face, and variation-setting APIs on HarfBuzzSharp.Font, along with new tests and updated generated bindings.

Changes:

Add SkiaSharp variable-font APIs (SKTypeface.GetVariationDesignParameters, GetVariationDesignPosition, CloneWithArguments) plus tag helper utilities.
Add HarfBuzzSharp variable-font APIs for axis queries, named instances, and setting/getting variation coordinates; enable generation for hb-ot-var.h.
Add SkiaSharp and HarfBuzzSharp tests covering variable-font behavior; update the generator’s include-path discovery for Linux.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
utils/SkiaSharpGenerator/BaseTool.cs	Adds Linux system/Clang include discovery for header parsing.
tests/Tests/SkiaSharp/SKVariableFontRenderingTest.cs	New rendering-based tests validating variable font rendering differences and clone stability.
tests/Tests/SkiaSharp/SKTypefaceTest.cs	Adds SKTypeface-level tests for variation axes/position and cloning.
tests/Tests/HarfBuzzSharp/HBFontTest.cs	Adds HarfBuzz `Font` tests for setting variations, coords, and named instances.
tests/Tests/HarfBuzzSharp/HBFaceTest.cs	Adds HarfBuzz `Face` tests for variation axes, named instances, and presence checks.
binding/libHarfBuzzSharp.json	Stops excluding `hb-ot-var.h` so bindings for var APIs can be generated.
binding/SkiaSharp/SkiaApi.generated.cs	Adds P/Invokes for Skia typeface variation APIs and new structs.
binding/SkiaSharp/SKTypeface.cs	Exposes the new variable-font APIs on `SKTypeface`.
binding/SkiaSharp/SKFontVariationAxis.cs	Adds tag helpers (`SKFontVariationTag`) and readable tag-name properties on new structs.
binding/HarfBuzzSharp/HarfBuzzApi.generated.cs	Adds P/Invoke surface for `hb-ot-var.h`.
binding/HarfBuzzSharp/Font.cs	Adds variable-font setters/getters on `HarfBuzzSharp.Font`.
binding/HarfBuzzSharp/Face.cs	Adds variable-font axis/named-instance query APIs on `HarfBuzzSharp.Face`.

mattleibow · 2026-04-22T01:34:35Z

/azp run

azure-pipelines · 2026-04-22T01:34:41Z

No pipelines are associated with this pull request.

mattleibow · 2026-04-22T02:11:58Z

Hey @ramezgerges, quick question: did you run pwsh ./utils/generate.ps1 to get this diff or was it something else?

Since we never got a chance to update the harfbuzz API bindings to the newer HarfBuzz version, I have been just bumping the underlying harfbuzz DEPS version and not re-generating the API. The changes now are so big that the work is a bit harder to do. So many weird types and things. What I usually do is drop back to 2.8.2 and the run the generator.

# In externals/skia/DEPS, revert harfbuzz back to the version we last generated from:
# Change: 2b3631a866b3077d9d675caa4ec9010b342b5a7c
# To:     63e15eac4f443fa53565d1e4fb9611cdd7814f28

dotnet cake --target=git-sync-deps
pwsh ./utils/generate.ps1

This will show us exactly what needs updating in binding/libHarfBuzzSharp.json so we can make sure the API is usable and not just direct mapping without the human touch.

There is a goal at some point to actually expose more, but never got to it. The version of harfbuzz being built is much later (still a bit old now) but not actually 2.8.2 - I think 8.x at least. There is a dream to one day bump harfbuzz and regen the full API and then be amazed.

ramezgerges · 2026-04-22T15:18:34Z

@mattleibow Yes, I ran the script for harfbuzz and took only the relevant parts. The PR does contain changes to the json files.

Native layer (mono/skia submodule): - C API: sk_typeface_get_variation_design_position, sk_typeface_get_variation_design_parameters, sk_typeface_clone_with_arguments - New structs: sk_fontarguments_variation_axis_t, sk_fontarguments_variation_position_coordinate_t SkiaSharp managed bindings: - SKTypeface: GetVariationDesignParameters, GetVariationDesignPosition, Clone - SKFontVariationTag: Make("wght") / GetName(tag) helper - TagName property on SKFontVariationAxis and SKFontVariationDesignPositionCoordinate - Generated P/Invoke bindings and struct type mappings in libSkiaSharp.json HarfBuzzSharp managed bindings: - Face: HasVariationData, axis queries, named instance enumeration - Font: SetVariations, Set/GetVarCoords, SetVarNamedInstance - P/Invoke bindings for hb-ot-var.h (removed from exclude list) Tests: - HBFaceTest, HBFontTest, SKTypefaceTest, SKVariableFontRenderingTest Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

- Face: validate instanceIndex >= 0 on named instance methods - Face: explicit (int) cast on uint array lengths - Font: validate instanceIndex >= 0 on SetVarNamedInstance - Tests: dispose Blob in CreateVariableFace/CreateVariableFontPair helpers Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

azure-pipelines · 2026-04-23T01:44:52Z

No pipelines are associated with this pull request.

Interactive demo that showcases the new variable font APIs: - Weight axis slider (100–900) with real-time text rendering - Optical size axis slider (14–32) - Weight spectrum showing all 9 standard weights - Uses Inter Variable font (SIL OFL license, 2 axes: wght, opsz) Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

azure-pipelines · 2026-04-24T19:08:06Z